Anderson Greenwood AG-14 and the MARVEL program

by Paul R. Matt

In 1950 the Office of Naval Research initiated a program to study the effects of the boundary layer on aircraft flying surfaces, and the Aerophysics Department of Mississippi State University was awarded the contract. Initially the program delved into this mysterious but promising effect of controlling the flow of air over and under the surface of a wing. It subsequently evolved into an extensive and highly unorthodox pattern of values and devolpments. If the air being forced over the wings could be properly energized and made to follow the contours of the aircraft surfaces, aircraft of the future could be made to take off in a shorter distance, land slower, lift greater payloads on less power and fly further in the process.

For twelve years the Navy sponsored this research into the phenomenon called boundry layer control (BLC). In 1962, however, the Navy abandoned the project and the U.S. Army Transportation Research Command continued the vitally important work. Under new directives the Army was heing limited to the size and type aircraft it could use. The BLC program held promise of providing information useful to improving performance in the size restricted aircraft the Army was suddenly being forced to operate.

The first experiments undertaken by Mississippi State sought a method of stabilizing the laminar boundry layer by means of suction (one form of boundry layer control) through thousands of perforations, tiny holes, distributed over the wing skin. For this, the first aircraft procured was a surplus military Schweizer TG-3 glider.

Experimentation was also conducted into the use of suction in preventing separation of the turbulent boundry layer which forms on lifting surfaces operating at high lift coefficients. These tests were also performed on the TG-3 glider.

The experiments with suction to prevent turbulent boundary layer separation were so successful that the second phase was initiated almost immediately. A powered machine, a Piper L-21 "Super Cub", was purchased. Studies with this plane showed a marked delay in air separation over the wing when boundry layer control was applied .This led to further aero-dynamic studies from which the acquired knowledge was eventually translated into present day high lift systems, enabling even large aircraft to operate from short or confined airfields.

These first two aircraft were fabric covered and their skin was too unstable to provide full BLC test values. A second generation of high lift modifications was then undertaken with an all metal Cessna L-19 "Bird Dog" airplane. This plane had a more refined boundry layer system installed, including an automatic, hydraulically driven blower system to provide the necessary suction. Results with the L-19 were impressive but because of inherent limitations in the airplane itself the full potential of BLC still could not be realized.

During research on the L-19 it was found that the propeller slipstream, with its great turbulence and high flow irregularities, contributed significantly to the flow separation on the wing which in turn limited the lift that could be developed. A pusher aircraft, long recognized as potentially the most efficient type airplane designed, was expected to eliminate this difficulty. In addition, if the thrust benefits of a ducted propeller were to be utilized, the take off capabilities of the aircraft would be further enhanced.

Still further studies into the ducted or shrouded propeller principle brought the research to a point where a modern, safe and thoroughly reliable pusher type airplane was needed. The Aerophysics Dept. Iooked about and the only aircraft that was suitable for continuation of the Flight Research Program was the Anderson-Greenwood AG-14. The Dept. Iocated one of the machines and persuaded the owner to sell it to Mississippi State. On June 27, 1955, Serial Number 4, N 3903K was purchased from Perry R. Delaney, Jr. of Tampa, Florida.

Upon receiving the plane, engineers at Mississippi State initiated construction of a duct or shroud about the propeller. About 20 hours of flight time was put in using this configuration and great improvements were noted immediately in both static and low speed thrust of the airplane. It was the first time in-flight research had been carried out on a ducted propeller. Much experience was also gained in the study of vibration and mechanical problems arising from the ducted propeller concept.

Further research required different propellers to be utilized and a study of their effectiveness had to be made. A series of aluminum propellers were built to test the theories but since none of these specialized propellers were airworthy, the AG-14 was grounded for the remainder of the test period. Following the completion of this phase of the program, S/N 4, N 3903K was restored to its original configuration. Mississippi State still flys it regularly for short trips, photo missions and general utility work.

Toward the end of 1956 the data compiled by numerous trials with the AG-14, tests on a small propeller test stand and theoretical calculations showed significant statics and low speed thrust advantages for the ducted propeller. These studies also pointed out the fact that the thrust increase, due to the propeller shroud, decreased with increased airspeed. At cruise speeds an increase in drag, over an open propeller, could be found.

Thus the idea was born to design the duct so that it could serve as/or replace some other component of the machine and could then be used to utmost advantage. Also continued testing indicated that in cruising flight, lateral forces on this ring closely approached those required for the normal or more conventional tail surfaces.

Further studies indicated that a new wing would also have to be employed, if the full effects of a total BLC system was to be realized. To minimize negative air pressure peaks on the wing, a camber-changing system was devised for use instead of flaps, and fiberglass skin construction was specified to minimize surface irregularities. Irregularities resulting from rivets, skin joints and "oilcanning", associated with normal thin skin construction, have always made it more difficult to control the boundry layer over lifting surfaces. Cantilever construction and specially designed fillets were required to further reduce interference. The desing was fast evoling into a highly unorthodox aircraft, but such an advanced concept would be the only suitable vehicle which could fully evaluate the boundry layer control effects.

This new plane was designed under the Mississippi Aerophysics Research Vehicle Extended Latitude (MARVEL) program. The airplane was to be built from criteria specified in the latest engineering studies and to use parts of no existing aircraft. Power was to be a 250 shp Allison T63 turboprop engine, but the Aerophysics Dept. was forced to make a substitute proposal when they found that the powerplant would not be available in time to meet the proposed program schedule.

To at least prove the feasibility of the configuration it was decided to build a smaller, less powerful, intermediate, aircraft. For this purpose the best suited airframe available was again the AG-14. Since extensive modifications were envisioned, a second machine was needed. On january 29, 1957, S/N 2, N 3901K, was acquired from Mr. C. E. Ragland of Brunswick, Georgia. The original plan for this plane was to remove the tail and tail booms, add on an aft fuselage section and a ducted framework around the propeller which would be driven through a long fiberglass extension shaft. The wings would be clipped and a chamber-changing section provided behind the spar of the existing wings. This modification of the AG-14 received the military designation of MA18 (Mississippi Aerophysics-study 18).

Plans to build this configuration were subsequently abandoned in favor of a more drastically modified version, one which would more closely reflect the final MARVEL goal. Originally this version was to carry the MA-18B designation and the nickname Marvelette - identifying it as the petite first generation vehicle of the bigger, more powerful Marvel machine of the future. In converting N 3901K to the Marvelette, the registration number was cancelled on January 18, 1960, and the converted aircraft was subsequently redesignated as the U.S. Army's XAZ-I. In the transformation, the AG-14 fuselage pod, engine, engine mounts and landing gear were all that was retained. Special BLC Marvel wings were constructed and replaced the AG-14 panels. These -14 wings are still in storage at Mississippi State and it is intended to keep them in case of damage to S/N 4 still on the airworthy flight line.

In addition to the Marvel BLC wing system, the tail surfaces were replaced by an extended propeller shroud which also functioned as the conventional straight vertical and horizontal tail arrangement. This airfoil shaped ring had articulated trailing edge segments that served as control surfaces. Two segments on either side acted as rudders, and four segments, four on top and four on bottom, acted as elevators. The special BLC system and the ducted propeller configuration formed the only practical approach to a complete propeller powered BLC aircraft.

The Marvelette was flown on numerous occasions and provided all the early evaluation data needed to prove the Marvel concept. The articulating control surfaces, however, proved to be less than desirable due to high friction and limited control power.

When the flight test program on the XAZ- I Marvelette was completed the Aerophysics engineers continued to use the configuration for static ground tests but at the conclusion of the program, the cost of restoring the aircraft to its original configuration proved too high. Rather than scrap the airframe, MSU placed the XAZ-1 in storage in case other research projects might want to utilize the unusual vehicle in the future.

The work accomplished on the XAZ-I proved the feasibility of the various concepts incorporated in its configuration and resulted in the construction of the XV-11A Marvel; Aerodynamic Research Aircraft.

This aircraft is constructed entirely of glass-reinforced plastic, utilizing a distributed suction boundry layer control system, variable wing camber and a rear mounted ducted propeller. Power is provided by a T-63 turboprop engine developing 270 shaft horsepower. A performance and stability and control flight test program on this aircraft was completed in 1970 for the U.S. Army. Although the aircraft is still in flying condition, failing interest in STOL aircraft of this type by the Department of Defense has curtailed research activities at the present time.

The author expresses his appreciation to Mr. Melvin Swartzberg of Mississippi State University for making the technical reports on the MARVEL program available for this capsulated presentation.

Captions:

Photo 1
AG-14, s/n 4 used by Mississippi State University to evaluate ducted propeller principle and other BLC/STOL values. Plane on tuff control flight. Performance was improved greatly. [Mississippi State University]

Photo 2
Cessna Aircraft testing same ducted propeller principles on their highly stylized swept-wing XMC model, (1972-73). The design and basic concept is remarkably similar to the AG-14 and Mississippl State development accomplished twenty years eariler. [Cessna Aircraft]

Photo 4 , Photo 5
End result, to date (1973), of Mississippi State University's study on BLC/STOL aircraft - the MARVEL Program. This is the MA-18B (Mississippi Aerophysics design 18) "Marvelette", adopted as the first generation test bed of the MARVEL contiguration. It incorporates the fuselage of AG-14, s/n 2, N3901K, with special wings and annular tail section. The U.S. Army designated the converted aircraft XAZ-1. [Mississippi State University]

Drawing 1
Mississippi State MA-18B "Marvelette" developed from AG-14 fuselage pod, power package and landing gear. [Mississippi State University]

Drawing 2
Below: the MA-17 MARVEL, final configuration adopted as the ultimate in a propeller driven STOL/BLC research vehicle. Power was to have been a 270 shp Allison Turboprop engine. This program reached full engineering compilation and wind tunnel model stage but was shelved when interest and funding by the military ceased and the contract cancelled. [Mississippi State University]